The long term goal of this program is to enhance understanding of the factors determining the disposition and metabolism in animals of lipophilic, water-insoluble xenobiotics. These substances are highly soluble in the lipids of membranes; so the solvation forces determining mechanisms of entry into cells, dispersion to metabolic systems within cells, and specific interactions with enzymes of detoxification will be different for lipophilic versus hydrophilic substances. This aspect of the metabolism of xenobiotics tends to be ignored. Yet, whenever studied, the data indicate that the metabolism of water-insoluble substances cannot be looked at in the context of what is known for water-soluble substances. The experiments in this proposal are aims specifically at providing information on how the lipids of membranes influence the distribution of lipophilic substances between tissues, how the lipids of membranes influence the partitioning of water-insoluble substrates between membrane lipids and the active sites of membrane-bound enzymes, and how these lipids influence the rate of entry of water-insoluble substances into tissues. The specific experimental approaches to these questions are determination of differential solubilities of water-insoluble compounds like fatty acids, benzpyrene, and bilirubin in membranes with different lipid compositions, the rates of transbilayer movement of these compounds and the rates of hydration when these compounds are bound to bilayers. In addition, we will examine the pathway for formation of E.S complexes between membrane-bound enzymes (acyl CoA ligase, P450, UDP-glucuronosyltransferase) and substrates that disolve readily in membranes. These experiments will be carried out using pure delipidated enzymes that have been reconstituted into lipid membranes of defined constitution. Finally, we will examine whether the uptake of water-insoluble substances by liver requires specialized mechanisms or whether it occurs because of the physcial chemical properties of the substance of interest, lipid membranes, and albumin. Tissues from rat and rabbit will be used in the course of these experiments.